This study will determine the relative lung fluid concentrations obtained following a single dose of rhDNase delivered by mask inhalation in a group of CF patients less than 5 years of age. It will also determine the concentrations achieved by mask inhalation in this group compared with children aged 5-10 years in whom this medication has been shown to be safe & effective when delivered by an oral mouthpiece. Also it will determine that a single dose of aerosolized rhDNase by mask for 14 consecutive days is safe in children with CF <10 yrs.